Generally the lubricant is used at the sliding portion and the rotational portion of cars and most machines represented by industrial machines. The lubricant is classified into a liquid lubricant and a solid lubricant. Grease having a configuration-holding performance brought about by thickening lubricating oil and a solid lubricant which holds the liquid lubricant and is thus capable of preventing the liquid lubricant from scattering and dripping are known.
For example, known solid lubricants (see patent documents 1 through 3) formed by mixing the ultra-high-molecular-weight polyolefin or the urethane resin and the hardener with the lubricating oil or the grease have the property of gradually exuding the liquid lubricating component with the liquid lubricating component kept retained between molecules of the resin. The self-lubricating polyurethane elastomer formed by allowing reaction between the polyol which is a polyurethane material and the di-isocyanate in the lubricating component in the presence of the lubricant is also known (see patent document 4).
When these solid lubricants are enclosed in a bearing and solidified, the lubricating oil is gradually exuded therefrom. These solid lubricants are intended to eliminate the need for maintenance for replenishing the lubricating oil and be utilized to prolong the life of the bearing in a strict environment where a large amount of water is present and environment where a high inertial force acts.
In recent years, cars have come to have a high performance, be compact, and be lightweight. Thus a constant velocity universal joint used to transmit a driving force is increasingly demanded to have a long life in addition to the above-described performances. Because cars have become compact and lightweight, a high load is applied to the constant velocity universal joint. Therefore there is a case in which lubrication with the conventional grease is incapable of allowing the constant velocity universal joint to have a sufficiently long life. Because cars will be demanded to have a higher performance in the future, it is difficult to fully cope with the above-described problems by merely optimizing the enclosing amount of the grease and the kind of the additive. Therefore there is a growing demand for research and development of new lubricants and lubricating mechanisms to be used for the driving portion and the like of the constant velocity universal joint.
Regarding the above-described demand, the conventional solid lubricants proposed in the patent documents 1 through 4 have a large lubricant-holding force, but have problems that they lack flexible deformability. When the solid lubricant is used at portions such as the driving portion of the constant velocity universal joint where an external force such as compression, flexure, and the like is repeatedly applied at a high frequency, a very large force is necessary to deform the solid lubricant in conformity to the compression, flexure, and the like. Because a very high stress is applied to the solid lubricant, it is necessary for a portion retaining the solid lubricant to have a high mechanical strength.
The strength of the solid lubricant and the filling rate thereof make compensation for each other. Therefore it is difficult to hold the lubricant at a high filling rate. Thus there is a possibility of preventing the constant velocity universal joint from having a long life. Therefore there is a demand for the development of a solid lubricant which can be easily used at the portion to which the external force such as compression, flexure, and the like is repeatedly applied at a high frequency.
To comply with such a demand, the foamed (solid) lubricant is known in which the flexible resin foamed to form communicating pores therein is impregnated with the lubricating oil so that the lubricating oil is retained in the communicating pores (see patent document 5). The solid lubricant is compressed in conformity to the boot which is deformed by the flexure of the constant velocity universal joint. The liquid lubricant which has exuded from the solid lubricant owing to compression is supplied to a necessary portion to allow preferable lubrication.
But the method of impregnating the resin with the lubricant disclosed in the patent document 5 is of a later-impregnation type of impregnating the foamed resin with the lubricating oil. When the above-described foamed (solid) lubricant is used, it has a low lubricating oil-retaining force because the lubricating oil is not contained in the solid component. Thus the above-described impregnation method has a defect that the lubricating oil goes out at once from the foamed resin, when the foamed (solid) lubricant is used in a high-speed operation condition. The above-described material can be used for lubrication in a short period of time and in a closed space. But it is difficult to use the material for a long time and in an open space. In addition, because the material does not have a high oil-retaining force, the lubricant always flows in the space, while the discharge of the lubricating oil and the absorption thereof to the foam are repeated. In such a case, in dependence on the chemical property of the lubricant and that of an additive contained therein, there is a possibility that they attack and deteriorate the boot material of the universal joint. Thus the foamed lubricant is defective in that one of the materials is limited to a specific kind. In addition, it is impossible to avoid a later impregnation-caused increase in the number of production steps, an increase in the production period of time, and consequently an increase in the cost.
For the above-described reason, there is a demand for the development of the foamed (solid) lubricant having a high force of retaining the lubricant and permitting a large deformation. It is particularly necessary for a solid component to contain the lubricant therein to enhance the force of retaining the lubricant.
Such a foamed lubricant can be supplied to a necessary portion in a necessary amount better than grease lubrication industrially widely used. Therefore the art of allowing the solid component to contain the lubricating component therein has advantages of decreasing the cost owing to a decrease in the use amount of grease, decreasing a load to be applied to the material for the boot, decreasing the weight of the constant velocity universal joint, and making the constant velocity universal joint compact. Thus it can be safely said that this art is high in the degree of importance for society not only from the standpoint of an economic side, but also from the standpoints of a decrease in a load to be applied to environment and an increase in the degree of freedom in design.
Even in the constant velocity universal joint in which the foamed lubricant having the above-described many advantages is enclosed, in dependence on the manner of using them, there is a case where a discharge amount of the lubricant is small due to an external force and a rise of temperature. In consideration of durability, it is desirable that the discharge amount of the lubricating component from the resin component is necessity minimum. When the discharge speed of the lubricating component is low, a necessary amount of the lubricant reaches the sliding portion at a low speed. Thereby the lubricant runs dry, which may cause wear and defective lubrication to occur at the sliding portion.    Patent document 1: Japanese Patent Application Laid-Open No. 6-41569    Patent document 2: Japanese Patent Application Laid-Open No. 6-172770    Patent document 3: Japanese Patent Application Laid-Open No. 2000-319681    Patent document 4: Japanese Patent Application Laid-Open No. 11-286601    Patent document 5: Japanese Patent Application Laid-Open No. 9-42297